This invention relates to improvements in the manufacture of microcellular foam tires particularly non-pneumatic or flat-proof tires.
Much work has been done since the early 1970's to overcome the inherent drawbacks in pneumatic tires, primarily the risk and expense of flat tires, without sacrificing their performance characteristics. A significant amount of attention has been directed to the use of polyurethane materials to achieve this end.
One of the earlier methods, which has since grown into a mature industry, is to fill an existing pneumatic tire with polyurethane foam. Such technology usually involves a two component room temperature system. The components are metered and blended together and then injected into the tire or tube cavity. The chemicals react to form polyurethane foam which expands and fills the volume normally occupied by compressed air.
The resulting tire is virtually flat proof and as such it has found application in industrial equipment where durability is a primary consideration. The main drawbacks are cost, in that urethane foam filled tire assembly is more expensive than an air filled assembly, as well as a noticeable reduction in the performance characteristics of the tire for some applications. Compared to a pneumatic tire, the mass is higher and the speed is limited as the urethane foam builds up and retains heat under the continual flexing resulting from rolling.
Another approach used was to replace the tire and tube with a hollow urethane elastomer shell. The urethane was stiff enough to support the load while the elastomeric nature of the urethane combined with the hollow interior allowed the tire to deflect to provide shock absorbing properties. Examples of tires produced using this approach are outlined in Canadian Patents 1,067,807, 1,099,205, and 1,112,550.
Tires made of an elastomer require some method of preventing them from elongating and rolling off the rim in use, as materials soft enough to provide a cushioning effect have relatively low tensile moduli. Such methods normally involve embedding into the tire a flexible tension member constructed of a material with the appropriate tensile stiffness. The objective is to have the resulting tensile stiffness of the composite tire such that the load required to stretch the tire on and off the rim is higher than the loads encountered in use. This allows the tire to be mounted, yet prevents it from inadvertently rolling off in normal usage.
Another method which can be used to hold the tire on the rim is to glue the tire to the rim. Although this method is technically feasible, it is too inconvenient to be readily acceptable for general sale and use.
Materials used as tension members include natural and synthetic fibre such as nylon, polyester and cellulose, and metal wires or coils. The general practice of using tension members in one form or another dates back at least to the beginning of this century.
A more recent innovation was to manufacture the tire from self-skinning flexible microcellular polyurethane foam. In this concept the foam interior provides the flexibility for the shock absorption characteristics, while the unfoamed skin on the outer surface provides the wear resistant riding surface. Naturally this concept maintains the "flat proof" characteristics by eliminating the need for compressed air. The tires are held on the rim through the use of tension members or glue as described earlier. A complete description of the concept can be found in Canadian Patent 1,032,455 and a detailed disclosure of a suitable urethane foam formulation is given in Canadian Patent 1,092,296.
Polyurethane foam tires are molded. In order to mold a tire shaped object the mold should be filled under some pressure to prevent the surface defects which will result if the urethane foam is unable to displace all of the air in the mold. This pressure can be obtained by pressurizing the liquid urethane components and then injecting them into a sealed mold, or, alternatively, by spin casting the tire. The preferred method to produce polyurethane foam tires is by spin casting as this is far simpler and more economical than injecting the liquid urethane under pressure.
Spin casting has been used extensively to produce products from various materials for many years. For example, descriptions of machinery and methods for spin casting products from various polymeric materials can be found in U.S. Pat. Nos. 4,479,769 and 4,519,971. Similarly methods of spin casting tires from liquid polymers such as urethane can be found in U.S. Pat. No. 3,751,551 and Canadian Patents 790,493, 790,498 and 875,065.
In order to produce the large number of tires needed to fill market requirements, a large number of molds are needed. Although metal molds are most certainly feasible for the production of molded foam tires, such molds are very expensive due to the complexity of the mold itself and the detail required in the mold cavity for such features as the tire tread.
For some molding processes such as injection molding, metal molds are essential because of high molding temperatures and/or pressures. However it has long been industry practice to produce molds from castable polymeric materials such as silicone, epoxy or urethane whenever the quantity of the required molds and the molding conditions are suitable.
This practice is suitable for the mass production of polyurethane foam tires. Rather than producing a large number of expensive metal molds, it is more effective to produce a metal "master mold" and then use that mold to cast the required number of production molds. As both the pressure and temperature experienced by the mold are low when spin casting polyurethane foam tires, urethane elastomers are suitable for use as production molds.
In summary, the preferred version of the prior art for nonpneumatic tires is to spin cast polyurethane foam tires with embedded tension members utilizing urethane molds.
The economics of spin casting polyurethane foam tires is such that the polyurethane materials are the largest single cost. It is therefore critical that the specific method used to produce the tire minimize the need for any excess urethane material over and above that required within the tire itself.
In the practice of the prior art, three primary causes of polyurethane material wastage have been identified.
One major cause of excess waste material is that existing methods of producing tires cannot ensure full coverage of the tension member by the urethane foam without wasting a considerable amount of material. Preblended liquid urethane components expand as they react and form urethane foam. Thus, to completely fill the mold cavity with unfoamed material is extremely wasteful as a volume of foam equal to the degree of expansion would be forced out of the mold. However, when only enough material to form the tire is introduced into a rotating mold, the liquid congregates at the outside diameter of the mold. As a result, tension members nearer the inside diameter of the mold are not immersed in and "wetted" by the liquid materials. Instead, the tension members must be engulfed by the expanding urethane foam which by this time is already in a state best described as semi-solid, so molding flaws result. The difficulty of covering the tension member by this expanding semi-solid foam is increased if the tension member is located near a wall of the mold as the expanding semi-solid foam must force its way between the tension member and the wall.
Various patents have been issued which to one extent or another address the issue of embedding reinforcing materials in a foamed product. These tend to fall into two broad categories. Some of these patents involve products where there is no significant difficulty in adequately covering the reinforcing member, such as U.S. Pat. Nos. 4,437,257, 4,338,270, 4,029,037, 3,991,146 and 3,511,736. Thus, although these patents discuss the need for reinforcements and methods of suspending them in the molds, the issue of complete covering of the reinforcement does not exist, and therefore is not addressed. Other patents, such as U.S. Pat. Nos. 4,702,866 and 4,461,736 specifically address the issue of causing the expanding foam to fully encapsulate and cover the embedded materials. However they accomplish this by more or less sealing the port by which the liquid raw materials were introduced. By blocking this potential exit to at least a significant extent, the expanding foam inside the mold cavity is placed under pressure which forces the foam in and around the embedded reinforcements. However blocking the material entry gate after injection of the liquid urethane is not practical in the case of spin casting foam tires.
Therefore in order to ensure the tension members are adequately covered when spin casting foam tires, the practice required by the prior art is to place excess material in the mold. In general, the greater the amount of material placed in the mold, the more likely that the tension members will be adequately covered. However this practice is costly in terms of waste material and therefore involves a compromise between the amount of material wasted with each tire produced and the number of tires rejected due to incomplete coverage of the tension member. Regardless, considerable material and money is wasted.
Another difficulty with prior art practice is caused by the need to have the mold accurately centered while being rotated. When liquid urethane is poured into a rotating mold, the liquid is distributed by centrifugal force. This force distributes the material according to the center of rotation of the mold, which does not correspond to the center of the mold itself when the mold is not accurately centered. The net effect is that portions of the mold which are closer to the center of rotation will receive less material than those which are further away. Therefore, if the mold is not properly centered, it will be necessary to overfill the more distant portions of the mold in order to adequately fill the portions of the mold closer to the center of rotation. The net result is that if the mold is not properly centered, excess material must be poured into the mold to ensure complete mold filling and adequate coverage of tension members which are normally located at the inner circumference of the tire.
The difficulty in centering the mold is increased by the use of urethane molds. This is because multiple urethane castings produced from the same mold can easily vary in dimension by plus or minus one half of one percent. For a typical tire mold, this can mean variations of over 3 mm. Thus when, as is often the case, the mold is a ring with inside and outside diameters approximately matching that of the tire to be cast, there are no consistent mold dimensions to use when attempting to center the mold mechanicaly.
The third difficulty is related to how the liquid material is fed into the mold. The standard method used by the prior art is to feed the material in more or less at the center of rotation, and then let centrifugal force distribute it out to the mold cavity. However this also leads to waste material, in that some method of distributing this material to the tire cavity such as runners or a flat plate is required, and some material adheres to this part of the mold or molding equipment rather than flowing into the cavity. This material is obviously wasted.